In recent years, liquid crystal display devices have been used widely in various fields, as monitors for portable notebook-type personal computers, display units of liquid crystal television receivers or liquid crystal television receivers with video recorder, and the like. A liquid crystal display device is basically constituted by a backlight unit and a liquid crystal display unit. In most cases, the backlight unit is of edge-light type. This is because the edge-light type helps to make the display device compact. The conventional backlight unit includes a light guide shaped like a rectangular plate and a linear or rod-shaped primary light source. At least one end face of the light guide functions as light-incoming face. The primary light source is, for example, a linear fluorescent lamp and extends along the light-incoming face of the light guide. The primary light source emits light, which is incident on the light-incoming face of the light guide, introduced into the light guide and emitted from one of the two major surfaces of the light guide, i.e., the light-outgoing face.
In recent years, it has been demanded that liquid crystal display devices having a relatively small screen, e.g. portable electronic apparatuses such as cellular telephones and portable game apparatuses or indicators on various electric and electronic apparatuses, should be small and consume but a little power. To reduce the power consumption, light-emitting diodes (LEDs), which are spot light sources, are used as primary light source in backlights. As disclosed in JP-7-270624-A (Patent Document 1), a backlight using LEDs as primary light source has a plurality of LEDs that are arranged in one-dimensional array along the light-incoming face of a light guide so that the backlight may perform the same function as a backlight having a linear light source. If such a backlight including a plurality of LEDs arranged in one-dimensional array is used, a desired amount of light and luminance uniformity over the screen can be attained.
It is demanded that the power consumption in such a small liquid crystal display be reduced further. To meet this demand, the LEDs used must be decreased in number. If less LEDs are used, however, the distance between the light-emitting points of the primary light source will become longer. The region of the light guide adjacent to the region between the light-emitting points will expand, inevitably decreasing the intensity of the light emitted from such a region of the light guide in a desired direction. This results in non-uniformity (i.e., luminance non-uniformity) of distribution in luminance in a viewing direction with respect to the light-emitting surface of the surface light source device.
JP-7-27137-5 (Patent Document 2) proposes a method in which a light guide having a rough light-outgoing face is used, and a prism sheet having an array of prisms is laid on the light-outgoing face of the light guide, with the prism face opposing the light guide, thereby to reduce the power consumption of the backlight and limit the distribution of output light not to sacrifice the luminance so much. This backlight can indeed provide high luminance at low power consumption. However, luminance non-uniformity may be conspicuous, visually seen through the prism sheet.
Of the luminance non-uniformity, the most problematical is such a shadow (dark region) as shown in FIG. 27, which develops in the light-guide at regions thereof corresponding to areas lying outside the LEDs 2 at the ends of an LED array or to areas between the adjacent LEDs 2. The dark region may be so large that it can be seen even in the effective light-emission region of the backlight, which corresponds to the screen of the liquid crystal display device. In this case, the backlight is greatly degraded in quality. The dark region will be more likely seen in the effective light-emission region if the number of LEDs used is decreased in numbers in order to reduce the power consumption, or if the distance between the LEDs and the effective light-emission region is shortened in order to make the display small. This luminance non-uniformity is inevitable, because the light bears emitted from the LEDs arranged adjacent on the light-incoming face of the light guide have directivity and diverge but a little in the light guide as they refract when being incident on the light guide. Further, what can be seen in the direction normal to the light-outgoing face are only the light beams that are substantially perpendicular to the prisms arrayed on the prism sheet. Inevitably, the light observed diverges lass than the light actually emitted from the light guide. Thus, the conventional backlight that uses spot light sources as primary light source can hardly achieve both a reduction in power consumption and the uniformity of luminance distribution.
In a backlight that uses, as primary light source, a linear light source such as a cold cathode-ray tube, the dark region in the vicinity of the light-incoming face may be illuminated by such a method as disclosed in, for example, JP-9-160035-A (Patent Document 3). In this method, the light-incoming face of the light guide is roughened. This method cannot sufficiently reduce the dark region in the backlight that uses spot light sources, such as LEDs, as primary light source.
JP-5-6401-U (Patent Document 4) and JP-8-179322-A (Patent Document 5) propose backlights having a linear light source such as a cold cathode-ray tube. In these backlights, many prisms extending in a direction perpendicular to the light-incoming face are arranged in an array on the light-outgoing face or on the opposite face, for the purpose of converging the light coming from a light guide with respect to a direction parallel to the light-incoming face. In the light guide having such a prism array, the introduced light is internally reflected by the prism so as to increase or decrease an angle between a direction of the reflected light and direction of incident light on the light guide. Hence, the light introduced into the light guide converges in an extending direction of the prism. The luminance can therefore increase. If such a light guide is introduced into a backlight having LEDs, the light introduced into the light guide will diverge as it is, reflected by the prism array provided on the light guide. The light which diverges is emitted from the light guide substantially in a direction Perpendicular to the prism provided on the prism sheet. The light therefore looks as if distributed widely, as viewed through the prism sheet.
Prisms, each having a cross section having straight sides, may be formed on a light guide. In this case, the light introduced into the light guide will diverge with anisotropy toward a particular direction. Consequently, luminance non-uniformity will develop, in the form of such slant bright lines as shown in FIG. 28. Further, as shown in FIG. 29, the light beams emitted from the spot light sources overlap one another, increasing the luminance at a part where two adjacent beams overlap. This also results in luminance non-uniformity.
To eliminate dark regions between the primary light sources or at the corners, the light-incoming end face may be roughened as described above. In this case, the dark regions indeed become small. However, more prominent luminance non-uniformity develops in the form of such slant bright lines as shown in FIG. 30.
In order to eliminate such luminance non-uniformity, it is proposed that the surface of prisms formed on the light guide be roughened or the linear shape of the prisms be changed, as disclosed in JP-2004-6326-A (Patent Document 6). Even in any surface light source device using such a light guide, however, dark regions may develop in front of the spot light sources based on that the luminance increases at the positions where the light beams emitted from the spot light sources overlap one another as shown in FIG. 29. This depends on the size of the device, the number of spot light sources (e.g., LEDs) arranged, or the distance between the spot light sources.
Patent Document 1: JP-7-270624-A
Patent Document 2: JP-7-27137-B
Patent Document 3: JP-9-160035-A
Patent Document 4: JP-5-6401-U
Patent Document 5: JP-8-179322-A
Patent Document 6: JP-2004-6326-A